Microscopic particles self-organize into a rolling mob

Don’t worry, they stop when we turn the current off.

One of the nearly infinite number of astounding sights in the animal kingdom plays out when creatures get together in large groups. Whether it’s a flock of birds or a school of fish, the group can take on an identity of its own, moving in seemingly perfect coordination. People have long puzzled over how this wonderful choreography works.

The flocking pattern is not directed by some controlling leader; it emerges from the behavior of individuals. Researchers have found that they can simulate virtual swarms by making individuals follow a few simple rules, such as keeping an equal distance from one’s neighbors. This research is no longer limited to the digital realm, though.

Antoine Bricard, Jean-Baptiste Caussin, and three of their colleagues have come up with a unique setup to build on previous studies. They used electrical insulators to create tiny spheres just 5 microns across (less than the width of a human hair) and placed them in a conductive fluid sandwiched between two microscope slides. Applying an electric field results in something called “Quincke rotation” in which uneven, fluctuating charges on the surface of the sphere interact with the field to make it spin. Turn on the field, and all these little particles start rolling around in random directions.

That is, until you increase the number of spheres beyond a threshold of crowding. That’s when it gets really interesting.

The researchers started by creating little “racetracks” for the spheres to roll around on. Once the density of particles is high enough, they begin to get together and move in the same direction—to flock. You can see a passing herd in the first video below.

The beads chase each other around a stretch of a circular racetrack.

Given a square enclosure in which to roam instead of circular racetracks, the spheres settled into a very different pattern. The constant motion hindered by collisions with the four walls generated a turbulent vortex, seen in the second video.

In a square-shaped arena, a complex pattern of behavior emerges.

There are two interactions that the researchers think are enabling the particles to put on these displays. Because of the uneven charge on the surface of the spheres, the spheres distort the electric field in their vicinity. That distortion acts to repel neighboring particles. The spheres also disturb the fluid around themselves as they move, nudging nearby particles to travel in a similar direction.

If particles only encounter these interactions occasionally, they won’t have any effect on the randomness of their motion. In a dense crowd, however, there's enough to build a critical mass of organization, creating a flock that moves as a unit.

Previous efforts to study flocking with physical experiments have used a variety of particles (including bacteria), but actual collisions between them were the most important way they interacted. Because the tiny spheres in this case interact in subtler but more predictable ways, the researchers believe their design will enable them to study “collective motion in more complex environments relevant to biological, robotic, and social systems.”